Blood cells are produced in the bone marrow, differentiated from immature cells, grown matured, and migrate to peripheral blood. In healthy adults, immature leukocyte does not appear in peripheral blood, however, immature leukocyte may appear in peripheral blood in patients with leukemia, metastatic bone marrow cancer, severe infectious disease. Therefore, it is extremely important to determine mature leukocytes and immature leukocytes in a biological sample for diagnosis of above-mentioned disorders.
As reagents for leukocyte determination, those disclosed in U.S. Pat. No. 5,958,776 are known. When a measurement sample in which the reagent and a biological sample are mixed is introduced into a flow cytometer and a light with specific wavelength is irradiated to obtain optical information, it is possible to classify mature leukocytes and immature leukocytes in the specimen based on the optical information and to count them, respectively. Further, it is possible to divide immature leukocytes into myeloblast and immature granulocyte and to count them, respectively. However, when a sample containing immature leukocytes was processed using this reagent, damage of immature leukocytes such as myeloblast was promoted in some cases, depending on processing conditions thereby resulting in deterioration of the classification accuracy. Therefore, in order to execute accurately classification or counting of immature leukocytes in the sample using the reagent, it was necessary to observe stringent control of processing conditions such as reaction temperature, reaction time or the like.